1. Field of the Invention
The present invention relates to railway air system air dryers and, more particularly, to an air dryer having a heating control system for preventing freezing of valves.
2. Description of the Related Art
A typical “twin-tower” desiccant-type air dryer includes two drying circuits that are controlled by valves. Wet inlet air flows through one circuit to remove water vapor, while dry product air counter flows through the other circuit to remove the accumulated water and regenerate the desiccant. Inlet and outlet valves for each pneumatic circuit are responsive to controlling electronics to switch the air flow between the two circuits so that one circuit is always drying while the other is regenerating. The air dryer may include a pre-filtration stage with a water separator and/or coalescer positioned upstream of the drying circuits. The pre-filtration stage removes liquid phase and aerosol water and oil that can accumulate in air supply system as a result of the compression of ambient air by the locomotive air compressors. A pre-filtration stage includes a drain valve that is used to periodically purge any accumulated liquid. For example, a typical pre-filtration drain valve actuation cycle might command a purge (open) for two seconds every two minutes.
The air dryer valves, including any pre-filtration drain valve, are constantly subjected to wet air and thus prone to freezing at low temperatures. In order to counteract this problem, a heater element may be provided to warm the valves sufficiently to prevent freezing. Unfortunately, it takes time to sufficiently warm the valves when the air supply system is powered up from a cold temperature. If any of the valves are commanded open before they are sufficiently warmed, the valves can freeze in the open position. If a valve remains in an open position when it should otherwise be closed, there is a risk of an uncontrollable venting of the compressed air from the locomotive air supply system. Further, due to the high volume of air flowing through the frozen valve, the heater may not have sufficient power to thaw the frozen valve, if it is frozen open. Thus, there is a need for a heating control system that ensures that the valves are sufficiently warmed before they are operated so that they do not freeze.